1. Field of the Invention
The invention relates to a method for removing ambient illumination from an image to allow further processing or analysis of the image.
2. Background of the Invention
There are several methods known as biometrics for recognizing or identifying an individual. These methods include analyzing a signature, obtaining and analyzing an image of a fingerprint and imaging and analyzing the retinal vascular patterns of a human eye. Recently the art has used the iris of the eye which contains a highly detailed pattern that is unique for each individual and stable over many years as a non-contact, non-obtrusive biometric. This technique is described in U.S. Pat. No. 4,641,349 to Flom et al. and U.S. Pat. No. 5,291,560 to Daugman. The iris identification techniques disclosed by Flom and Daugman require a clear, well-focused image of the iris portion of the eye. Once that image is obtained a comparison of that image with a coded file image of the iris of the person to be identified can be accomplished quite rapidly. However, prior to the present invention there has not been an optical system which could rapidly acquire a sufficiently clear image of an iris of the person to be identified unless that person positioned his eye in a fixed position relatively close to an imaging camera. In a commercial embodiment of this iris identification the user is required to position his eye on an eyepiece. This provides an advantage in that the eyepiece eliminates the effects of ambient lighting. It also eliminates the necessity of automatic iris image acquisition, since the subjects themselves are responsible for positioning their own eyes at the proper location for the device to function. However, the device is impractical for users of automated teller machines and for other situations in which an individual must be rapidly and unobtrusively identified. Yet, when one allows the person to be identified to stand away from the camera lens, ambient lighting conditions such as background lighting can confuse and distract the process of automatic iris acquisition, depending on the algorithms used. In addition, lighting from around the subject may obscure portions of the image or create artifacts that prevent identification.
It has been known for many years that the effects of lighting could be removed by taking two images, one while the illuminator is on and the second while the illuminator is off. Then a pixel by pixel comparison is made between the two images. Usually this has involved subtraction of the gray scale values for corresponding pixels in the two images. Sometimes pixel values have been enhanced by multiplication of the gray scale values or by other mathematical operations. Furthermore, the illuminators could generate visible light, infrared light beams or even X-rays to create the image. Examples of such prior art image subtraction are contained in U.S. Pat. Nos. 4,636,850 and 4,490,037.
One problem with the prior art image subtraction methods is that one must know which of the two images was made while the illuminators were on and which was made while the illuminators were off. A second shortcoming of the prior art is that most of the techniques require a relatively large memory to store the images and the processing algorithms. Finally, many of the techniques are difficult or impossible to use with video cameras.
There is a need for an image subtraction technique which overcomes these shortcomings of the prior art. This system would be particularly useful to identify users of automated teller machines as well as individuals seeking access to a restricted area or facility or other applications requiring user identification. However, for such use to be commercially successful, there must be a rapid, reliable and unobtrusive way to obtain iris images of sufficient resolution to permit verification and recognition from an ATM user standing in front of the teller machine, and for the obtaining images of the subjects themselves to enable automatic iris image acquisition. To require the user to position his head a predetermined distance from the camera, such as by using an eyepiece or other fixture or without fixturing is impractical. Hence, the system must rapidly take and process the images so as to eliminate effects caused by movement of the subject.